If this is the only app that doesn't work for a while why not use a Virtual machine with an OS that it does work on -- VISTA, Fedora, XP etc.
The overhead of a VM on a W7 X-64 with decent RAM is minimal if your VM is only running a few applications (or a single one in this case).
That way you can continue to enjoy W7 and still run Mathematic 7 whne you need it -- actually if you can get it to run on XP you can allocate as little as 768 MB (or even 512MB) RAM for your VM. Your Host 64 bit W7 won't even notice this if you've got even a half way decent processor -- mot are dual processors anyway. (BTW only allocate a SINGLE processor to the "Virtal machine" Virtual hardware set. Things like VBOX (free), Virtual PC (free) and vmware workstation (pay for) have "Virtual Machine Wizards" for easily setting up a Virtual, machine and you can share disks to your real machine -- so you can save your data.
I prefer vmware workstation but it's a little bit pricey -- however if you use VMWARE PLAYER (FREE) you can use virtual machines created by vmware workstation.
If you download QEMU (FREE) you can actually create vmware format Virtual machines and then use vmware player to use the virtual machine. It's a bit fiddly to set up but you can then get all the advantages of being able to create and use Virtual machines like vmware workstation but without costing anything.
There are a number of methods for using QEMU easily to create virtual machines for use in vmware player .
This one creates a Windows XP basic VM but you can fiddle around and google on how to use QEMU to create VM's for other OS'es.
Once you've created the Virtual machine use vmplayer to run it. VMware Player with your own Windows XP Professional Virtual Machine